1. Field of the Invention
The present invention relates to a semiconductor pressure sensor assembly, and, more specifically, to a package structure of a semiconductor pressure sensor assembly.
2. Description of the Related Art
Semiconductor pressure sensors such as contact-type pressure sensors are well known in the art. For example, contact-type semiconductor pressure sensor chips made by lo diffusing a semiconductor piezoelectric crystal resistor, i e., a "distortion gauge resistor", on a thin diaphragm formed in a portion of a silicon substrate are well known. In such a semiconductor pressure sensor chip, pressure applied to the chip causes the resistance of the distortion gauge resistor to vary, and such changes in resistance are converted into an electric signal which can be used to measure the pressure.
FIGS. 5-6 illustrate a conventional assembly of the above-described semiconductor pressure sensor. As shown in FIGS. 5-6, the assembly comprises a semiconductor pressure sensor chip 1 mounted on a glass base 2, a plurality of bonding pads 1a formed on a top surface of the sensor chip 1, external conducting terminals 3, and aluminum wires 4 for connecting the bonding pads la to the external conducting terminals 3. As also shown in FIGS. 5-6, the assembly further comprises a receptacle 5 comprising, for example, a portion of a resin dual in-line package, having a cavity 5a recessed in its floor, a lid 6 having a pressure application hole 6a formed therethrough, and a silicone gel 7 for protecting the surface of the sensor chip 1.
The semiconductor pressure sensor chip 1 is hermetically joined to an upper surface of the glass base 2 such that a concave space of the substrate on which a diaphragm has been formed is directed toward a lower plane. The glass base 2 is joined, via an adhesive agent, at the floor of the cavity 5a. The external conducting terminals 3 are arranged about the periphery of the receptacle 5 so as to surround the cavity 5a and the sensor chip 1. It should be noted that it is desirable that the depth of the cavity 5a be such that the height of the sensor chip 1 when mounted on the glass base 2 within the cavity 5a and the height of the external conducting terminals 3 are substantially the same within the receptacle 5. It should also be noted that the silicone gel 7 protects the sensor chip 1 and the aluminum wire 4 from dust and humidity which can reach the sensor chip 1 through the pressure application hole 6a.
The method for providing the assembly illustrated in FIGS. 5-6 is as follows. First, the sensor chip 1 and the base 2 are assembled in the cavity 5a. Next, the aluminum wires 4 are bonded between the bonding pads 1a of the sensor chip 1 and the external conducting terminals 3. Next, a proper amount of the silicone gel 7 is potted into the receptacle 5 so as to cover the surface of the sensor chip 1 and the aluminum wires 4. Finally, after the silicone gel 7 has been cured, the lid 6 is attached to the receptacle 5.
It should be noted that, when the silicone gel 7 is potted into the receptacle 5, care must be taken to maintain the precision of the sensor chip 1. More specifically, because the silicone gel 7 still maintains a somewhat gelatin state even after being cured, pressure externally applied through the pressure application hole 6a to the sensor chip 1 is more or less attenuated by the silicone gel 7. Moreover, the dynamic characteristics of the silicone gel 7 will tend to vary when subjected to heat and humidity as a function of the thickness of the silicone gel 7, thereby altering the measuring precision of the sensor chip 1. Accordingly, to increase the precision of the sensor chip 1, it is desirable that the silicone gel 7 be made as thin as possible.
In an effort to minimize the thickness of the silicone gel 7, Applicants have attempted to remove excess amounts of the silicone gel 7 from the receptacle 5 before the silicone gel 7 is cured using a vacuum device having a specially adapted suction nozzle 9. More specifically, the suction nozzle 9 of the vacuum device is positioned within the circumference of the sensor chip 1 such that the silicone gel 7 is removed from the receptacle 5 until a layer of only several tens of micrometers of the silicon gel 7 is left to cover the surface of the sensor chip 1. It should be noted that, although the aluminum wires 4 may still extend above the upper surface of the remaining layer of the silicon gel 7, because the silicon gel 7 has adhesive characteristics, it is still possible for the surfaces of the aluminum wires 4 to be coated with the silicone gel 7.
A problem with the foregoing approach to minimizing the thickness of the silicone gel 7 in the conventional assembly of FIGS. 5-6 is that the positioning of the aluminum wires 4 and the external conducting terminals 3, in addition to the small size of the sensor chip 1, e.g., 3 mm.times.3 mm, make positioning of the suction nozzle 9 within the cavity 5a extremely difficult. In particular, it is practically impossible to position the suction nozzle 9 within the circumference of the sensor chip 1 without touching the aluminum wires 4 and the sensor chip 1. As a result, the surface of the sensor chip 1 may be damaged and the aluminum wires 4 may be deformed or even disconnected.